A central office interface circuit is usually located on the subscriber's premises and is used to connect the subscriber loop to one or more subscriber apparatus units. The central office interface is generally designed to appear electrically to the central office, as a typical telephone set. It is also designed to isolate the DC component of the signal from the AC component of the signal at the subscriber's end. Since such a circuit is located at the subscriber's end of the loop there is not a battery feed provided at the interface circuit. A typical central office interface circuit particularly adapted for use with electronic key telephone apparatus is described in U.S. Pat. application Ser. No. 146,237 filed May 5, 1980 issued as U.S. Pat. No. 4,369,337 and assigned to the instant assignee.
The isolation of the DC component on the subscriber loop from the subscriber apparatus is most conveniently achieved by using an isolation transformer. However, such a transformer has to carry substantial current of up to approximately 120 milliamps on a short subscriber loop to the central office. In order to prevent saturation, the transformer has to be provided with a relatively expensive and fairly bulky laminated core with an airgap. In addition, since such a core generates significant fringing magnetic fields, the transformer must be shielded.
The solution to the problem of course lies in the provision of a circuit wherein the DC flux of the transformer is balanced to approximately a zero value. Flux balancing or cancellation is generally well known in the art for example as disclosed in U.S. Pat. No. 3,384,810, issued May 21, 1968 to E. S. Kelsey and assigned to the assignee of the instant invention. Other flux cancellation circuits are described in U.S. Pat. No. 3,959,718 and U.S. Pat. No. 4,103,112. These patents describe various serial and parallel transformer configurations which provide balance windings to substantially cancel the DC flux generated in the primary windings of the transformer in combination with means for connecting a source of power or battery feed to the subscriber loop. Basically, all the known prior art circuits operate by varying the DC current through the balance winding(s) to compensate for the DC flux generated by the current flowing through the primary winding.